The present invention relates generally to developers, development methods, development devices and their elements, and image-forming devices, and more particularly to a nonmagnetic and single component developer, a development method using the nonmagnetic and single component developer, a development roller, a blade regulating a thickness of a nonmagnetic and single component developer layer on the development roller, a method for forming a nonmagnetic and single component developer layer using the blade, a development device having the development roller and the blade, and an electrophotographic image-forming device having one or more of these elements. The present invention is suitable for a color laser printer, for example.
Hereupon, the xe2x80x9cnonmagnetic and single component developerxe2x80x9d is a single component developer that is not magnetized and includes no carrier. The xe2x80x9celectrophotographic image-forming devicexe2x80x9d, which is typically a laser printer, is a non-impact printer that provides recording by depositing a developer as a recording material on a recorded medium (e.g., printing paper and OHP film).
With the recent development of office automation, the use of electrophotographic image-forming devices such as a laser printer for computer""s output devices, facsimile machines, copiers, etc. have been spreading steadily. The electrophotographic process generally employs a photoconductive insulator (photosensitive drum), and includes the steps of charging, exposure to light, development, transfer, fixing, and other post processes.
The charging step uniformly electrifies the photosensitive drum (e.g., at xe2x88x92600 V). The exposure step irradiates a laser beam etc. onto the photosensitive drum and changes the electrical potential at the irradiated area down, for example, to xe2x88x9250 V or so, forming an electrostatic latent image. The development step electrically deposits the developer onto the photosensitive drum using, for example, a reversal process, and visualizes the electrostatic latent image. The reversal process is a development method that forms an electric field by a development bias in areas where electric charge is eliminated by exposure to light, and deposits the developer having the same polarity as uniformly charged areas on the photosensitive drum by the electric field. The transfer step forms a toner image corresponding to the electrostatic latent image on a recorded medium. The fixing step fuses and fixes the toner image on the recorded medium using the heat, pressure, etc., thereby obtaining a printed matter. The post processes may include a discharge and cleaning of the transferred photosensitive drum, a collection and recycle and/or disposal of residual toner, etc.
The developer for use with the aforementioned development step can be broadly divided into a single component system developer using the toner, and a binary component system developer using the toner and carrier. The toner may use a particle prepared, for example, in such a manner that a colorant such as a dye and a carbon black, or the like is dispersed in a binder resin made of synthetic macromolecular compound, and then is ground into a fine powder of approximately 3 through 15 xcexcm. A usable carrier may include, for example, an iron powder or ferrite bead of approximately 100 xcexcm in diameter. The single component system developer advantageously results in (1) simple and miniature development equipment due eliminating a carrier deterioration, toner density control, mixing, and agitation mechanisms, and (2) used toner without any waste such as a carrier.
The single component system developer may be further classified into a magnetic and single component developer that includes toner in a magnetic powder, and nonmagnetic and single component developer that does not include the same. However, the magnetic and single component developer is disadvantageous in (1) the low transfer performance due to the high content of low electrical resistant magnetic powder which hinders the increased electric charge amount, (2) the bad colorization due to its low transparent, black-color magnetic powder, (3) the low fixing performance due to the magnetic powder which requires high temperature and/or high pressure, increasing a running cost. Accordingly, the nonmagnetic and single component developer without these disadvantages is expected to be in increasing demand in future.
The nonmagnetic and single component developer commonly uses the toner having a relatively high volume resistivity (e.g., at 300 Gxcexa9xc2x7cm, etc.). In addition, the toner, as basically carrying no electric charges, needs to be charged by the triboelectricity or charge injection in the development device.
The development method employing the nonmagnetic and single component developer is divided into contact and noncontact development methods: The contact-type development method deposits a developer on the photosensitive drum by bringing the development roller carrying the developer into contact with the photosensitive drum; and the noncontact type development method provides a certain gap (e.g., of about 350 xcexcm) between the development roller and the photosensitive drum to space them from each other, and flies the developer from the development roller to and deposits the same onto the photosensitive drum. Disadvantageously, the contact-type development method may deteriorate the developer by friction between the development roller and the photosensitive drum, and besides cause crack the photosensitive film, shortening the life of photosensitive body. Accordingly, the noncontact-type development method without these deteriorations has recently been highlighted.
It is significant for the noncontact-type development process employing the nonmagnetic and single component developer to ensure a sufficient image density by controlling the amount of toner flying from the development roller to the photosensitive drum. Thus, it is important to form a toner thin layer while controlling its thickness on the development roller. As a typical method for regulating a toner layer thickness, it has conventionally been proposed to provide an elastic blade (restriction blade) in contact with the development roller to maintain the layer thickness uniform.
The development equipment applying the noncontact-type development method employing the nonmagnetic and single component developer generally comprises a reset roller, a development roller, and a blade. The development roller is connected with a bias power supply, and provided with the development bias of superposed AC and DC voltages from the bias power supply. The reset roller, which is also called a supply roller or application roller, contacts the development roller: The reset roller serves not only to supply the toner to the development roller, and but also to scrape off and remove the toner unused for the development and remaining on the development roller. The development roller, which is, for example, a roller made of metal such as aluminum, adsorbs the charged toner on its surface in the form of the thin layer, and conveys it to a development area.
The blade contacts the development roller and serves to regulate the toner layer to a uniform thickness. The blade may be made up of one elastic member such as urethane, or of a metal member having a contact portion made of resin with the development roller. For instance, according to Japanese Patent Publications (Kokai) Nos. 8-202130 and 6-102748, when a metal member, namely a rigid member, is used for the development roller, the toner layer may be regulated by bringing a blade made of an elastic body such as rubber into contact with the development roller; on the other hand, when a member made of an elastic body such as rubber is used for a surface of the development roller, the toner layer may be regulated by bringing an end portion or non-end portion (namely midsection) into contact with the development roller. In order to avoid damaging the development roller and the blade by mitigating the accuracy in contact pressure required at the contact portion between them, these prior art have devised to use a contact between one that is rigid and the other that is elastic. Japanese Patent Publications (Kokai) Nos. 8-202130 and 6-102748 also disclose a surface roughness of the development roller, a pressure with which the blade is pressed against the development roller (blade pressure), a toner particle diameter, and other conditions for forming a toner layer as shown in Table 1 below.
In operation, the toner is charged (e.g., negatively) by sliding friction among the reset roller, the blade, and the development roller. The negatively charged toner thereafter is fed onto a surface of the development roller by the reset roller, and deposited thereon by electrostatic adsorption. Subsequently, the toner layer on the development roller is leveled by the blade to form a thin layer having a uniform thickness of about 10 xcexcm through 40 xcexcm. The toner, which has been conveyed to a development area where a surface of the development roller is closest to the photosensitive roller, flies and adhered to an electrostatic latent image on the photosensitive drum with the electrical force of attraction using a predetermined voltage applied to the development area. Consequently, the latent image is visualized and developed. Next, the reset roller removes the residual toner on the development roller that is left in a no-image area where no latent image is formed. The development process repeats a series of these operations.
However, the conventional noncontact-type development method employing the nonmagnetic and single component developer would disadvantageously deteriorate the image quality depending on development conditions. The present inventors, as a result of their thorough study on what causes such image deterioration, have discovered that the image quality depends on the toner layer formation and toner""s electrical properties.
The toner layer would result, if too thin, in the low and uneven image density, while, if too thick, would increase a proportion of oppositely charged or low charged toner, thereby fogging the no-image area. The present inventors have discovered that the toner layer formation physically relies upon four parameters (though the parameters are not limited to these four) including a surface roughness of the development roller, a blade pressure, a toner particle diameter, and a charge amount of toner, and that these parameters should be controlled correlatively to some extent.
The inappropriate surface roughness of the development roller would constitute an obstacle to form a uniform toner layer. The surface roughness serves as a mechanical force for conveying the toner and as a spacer between the development roller and the blade. The surface roughness, if too small, would make a toner layer too thin and lower the image density, while, if too large, would make the toner layer too thick and thereby produce a fog in the no-image area (i.e., undesirably coloring with the toner an area which has no image and is therefore expected to be white clarity). In addition, if the blade pressure is too low, the toner layer thickness locally could not be regulated, and toner would easily escape from the blade. If the blade pressure is too high, toner would be so stressed as to produce a fusion of toner into the blade, and would be likely to deteriorate the toner charging. The toner layer thickness would vary with the toner particle diameter. Furthermore, if the toner charge amount is too large, the reflection force as the toner electric attraction force onto the development roller would increase and the toner layer would become thick. As a result, an increase of the blade pressure would raise a mechanical stress to the toner, and thereby tend to deteriorate the toner. On the contrary, if the toner charge amount is too small, the electric attraction force would be small and the toner layer would become thin. As a result, the blade pressure would need to be kept low, but this would cause the toner escape.
Furthermore, the instant inventors have discovered that the formation of the toner layer mechanically depends also upon (but is not limited to) a blade shape and a toner flow between the reset roller and the development roller. Now consider a conventional development device 1, for example, in which a blade 2 comprises a metal plate 2a and a rubber plate 2b stuck to the metal plate 2a, and comes into contact with a development roller 4 through the rubber plate 2b as shown in FIG. 8. Hereupon, FIG. 8 is a partially enlarged sectional view of the development device 1 employing conventional nonmagnetic and single component toner T. The conventional development device 1, as understood from FIG. 8, disadvantageously generates toner agglomeration TB at the top of the blade 2. The toner agglomeration TB occurs when the top of the blade 2 blocks, as shown by an arrow, the flow between the reset roller (not shown) and the development roller 4.
This toner agglomeration TB may locally apply the pressure between the blade 2 and the development roller 4, and would cause the excessive toner T to pass the blade 2 to the development roller 4, or on the contrary, to hinder the proper amount toner T from passing to the blade 2. Consequently, as shown in FIG. 9, the uniform toner layer may be unable to be formed. Hereupon, FIG. 9 is a partial schematic transverse section illustrating a state of the development roller 4 and the toner layer TL formed thereon. Failure to form a uniform toner layer would cause a degradation of image quality such as an uneven density and a white clarity as described above. Although the present inventors have considered processing of the rubber plate 2b into such a shape (e.g., as a bevel) that does not prevent the toner from flowing, they have discovered that the rubber plate 2b having a complex shape easily bent and altered its shape, increasing the difficulty to form a uniform toner layer TL. In addition, a blade made of an elastic member generally has disadvantages in difficulty in manufacturing, low durability, and high manufacturing cost.
On the other hand, with regard to the toner""s electrical properties, the present inventors have paid attention to toner""s volume resistivity. The present inventors have experimentally developed a filled-in image (such an image that a whole printable area is solidly filled) by employing commercially available or experimentally prepared toners having various resistance values. The instant inventors has resultantly discovered that, regardless of whether the toner has a uniform layer in thickness, use of some toners having certain resistance values in the noncontact-type development produced a stripe of black, or other colors if printed in multiple colors, in the paper feed direction, thereby deteriorating the image quality. The present inventors have assumed that this was because a high toner resistance value produces excessive charging in toner, which would lead to a dielectric breakdown inside the toner, causing uneven streaks.
Therefore, it is an exemplified general object of the present invention to provide a novel and useful developer, development method, development device and its elements, and image-forming device in which one or some of the above disadvantages are eliminated.
Another exemplified and more specific object of the present invention is to provide developer, development method, development device and its elements, and image-forming device that can stably form a high-quality image by a relatively inexpensive and easy means.
In order to achieve the above objects, a developer layer forming method according to one aspect of the present invention comprises the steps of charging nonmagnetic and single component developer having a volume average particle diameter D (xcexcm) to an average specific charge q/m (xcexcC/g), supplying said charged developer to a development roller having ten-point surface roughness Rz (xcexcm), and forming a layer of a single component developer having a layer thickness dt (xcexcm) on the development roller by providing a blade in contact with the development roller at a blade line pressure Pb (gf/cm), wherein dt, Pb, q/m and D meets the following relationships: 4xe2x89xa6Dxe2x89xa6xe2x88x9212, 5xe2x89xa6q/mxe2x89xa612, 1xe2x89xa6Rzxe2x89xa612, 20xe2x89xa6Pbxe2x89xa680, dt=1.8xc3x97{q/mxc3x97Rz/(Pbxe2x88x921)}xc2xdxc3x97Dxc2x10.25 D, and 1.5 Dxe2x89xa6dtxe2x89xa63.5 D. It has been experimentally demonstrated that a developer layer having a uniform thickness can be stably formed according to this developer layer forming method.
A development device of another aspect of the present invention comprises a development roller having a ten-point average surface roughness Rz (xcexcm), and a blade in contact with the development roller at a blade line pressure Pb (gf/cm), capable of forming a layer of a nonmagnetic and single component developer having a volume average particle diameter D (xcexcm) and an average specific charge q/m (xcexcC/g) on the development roller, the layer having a thickness dt (xcexcm), and dt, Pb, q/m and D meeting the following relationships: 4xe2x89xa6Dxe2x89xa612, 5xe2x89xa6q/mxe2x89xa612, 1xe2x89xa6Rzxe2x89xa612, 20xe2x89xa6Pbxe2x89xa680, dt=1.8xc3x97{q/mxc3x97Rz/(Pbxe2x88x921)}xc2xdxc3x97Dxc2x10.25 D, and 1.5 Dxe2x89xa6dtxe2x89xa63.5 D. It has been experimentally demonstrated that a developer layer having a uniform thickness can be stably formed according to this development device.
A development device of another aspect of the present invention comprises a metal development roller, and a blade contactable with the development roller at a predetermined blade pressure to form a layer of a nonmagnetic and single component developer on the development roller, wherein the blade includes a metal contact portion contactable with the development roller, the contact portion having a shape selected from a group consisting of sectionally acute-angled, curved, and round shapes. According to this development device, the contact portion having the shape in section of an acute angle, a curve, or a round can serve to prevent a toner agglomeration blocking a toner flow from forming and the toner from adhering and destroying.
A development device of still another aspect of the present invention comprises a metal development roller, and a blade contactable with the development roller at a blade pressure of 20 through 80 gf/cm to form a layer of a nonmagnetic and single component developer, wherein the blade includes a metal contact portion having a surface roughness less than a surface roughness of the development roller. According to this development device, a toner fusion to the blade can be avoided by controlling a surface roughness of the development roller and the blade, and the blade pressure.
A developer of one aspect of the present invention is usable for noncontact-type development process, comprises a colored fine particle and a fluidizing agent, and has a volume resistivity of about more than 10 Gxcexa9xc2x7cm but about less than 192 Gxcexa9xc2x7cm. A container of one aspect of the present invention stores the above nonmagnetic and single component developer. The nonmagnetic and single component developer has a resistance value experimentally evaluated as appropriate.
An image-forming device of one aspect of the present invention comprises a photosensitive drum, a charger which charges the photosensitive drum, an exposure part which exposes the photosensitive drum charged by the charger, and forms an electrostatic latent image, a development device which develops the photosensitive drum exposed, and visualizes the electrostatic latent image as a toner image, and a transfer part which transfers the toner image onto a recorded medium, wherein the development device comprises any of the above-described development devices. This image-forming device has the same effect as the above development devices.
An image-forming device as an exemplified embodiment of the present invention comprises a photosensitive drum, a charger which charges the photosensitive drum, an exposure part which exposes the photosensitive drum charged by the charger, and forms an electrostatic latent image, a development device including a development roller spaced apart from the photosensitive drum exposed, said development roller flying a nonmagnetic and single component developer to the photosensitive drum, developing the photosensitive drum, and visualizing the electrostatic latent image as a toner image, the developer having a volume resistivity of about more than 10 Gxcexa9xc2x7cm but less than 192 Gxcexa9xc2x7cm, a transfer part which transfers the toner image onto a recorded medium, and a container which stores the nonmagnetic and single component developer. According to this image-forming device, the nonmagnetic and single component developer has a resistance value experimentally evaluated as appropriate.